In vehicles, it is important from the point of view of safety that an operator operating (driving) with his or her eyes forwardly fixed to the front, when changing the visual point in an instant and looking at a display such as a velocity display, can accurately read out the displayed contents for the shortest possible time.
In recent years, in vehicles, particularly automobiles, there has been a strong movement to adopt a so-called "head up display (hereinafter referred to as a HUD)" which has been developed in aircraft. In this HUD, the displayed image of a CRT is projected on a combiner (semitransparent reflection plate) disposed at the front of a pilot's seat (or on a windshield) with an optical system such as lenses, mirrors, and holograms, and is displayed in a form superimposed upon the forward scene. As a result, the HUD becomes a unit which makes it possible to reduce movement of one's eyes or focusing as much as possible and to enhance visibility.
In the railroad vehicles, there is plenty of space for housing an image display section, and as shown for example in Published Unexamined Patent Application No. H4-110236, the image display section is comprised of considerably large-scale devices such as a CRT, a lens, a mirror, and a concave mirror.
On the other hand, in the automobiles, space for housing an image display section or an optical system is limited, and as shown for example in U.S. Pat. No. 4,740,780, U.S. Pat. No. 4,664,475 or "Development of Hologram Head-up Display" (SAE TECHNICAL PAPER SERIES 920600, (1992) by H. KATO et al.), liquid crystal or a LED is used as the image display section and is combined with an optical system such as a mirror, and the combined unit is housed in a compact form in an instrument panel. However, in order to form a focus at a great distance, the reflections at the mirrors are repeated to obtain a long optical path leading from the image display section to the combiner. For this reason, a certain space has become necessary. Thus, these optical systems are housed in the instrument panel, but it is desirable that the use of the space be avoided where various kinds of devices or wiring are close together, and if possible, it is desirable that the use of this space be avoided. That is, a conventional HUD such as this is an excellent display where the forward external scene and various kinds of image information and character information from the interior can be superimposed and viewed at front of the driver's seat, but in order to achieve this, the conventional HUD has a large problem that a considerable space must be occupied.
This problem originates in the fact that the HUD takes the projection type display structure that the image generated in the image display section is projected and displayed on the combiner surface. The HUD, as previously described, is originally a system which has been developed in aircraft, and various functions that had been required in that case, as they are, have been applied to vehicles, etc. Therefore, it is considered that the requirements which cannot always be said to be indispensable in vehicles have also been introduced as they are.
In aircraft, the forward view's space is a wide space where the field of view is not much obstructed, and the operator is operating focusing the visual point at the infinite point. Therefore, it is preferable that the image of the HUD be also focused at the infinite point. In addition, even if considerably complicated information were displayed on the display, it would not become an obstacle to the operation of the operator, and conversely, the operator does need such complicated information.
On the other hand, in vehicles, particularly automobiles, the driver is driving, viewing about 10 to 20 m ahead, about 50 m ahead at the most. Also, the forward external scene is full of variety, and the operator is looking at an ever-changing sight, such as scenes, various kinds of display plates, and forward vehicles. If extremely complicated information is displayed on the display, conversely it will become an obstacle to driving. When the driver looking about 20 m to the front looks at an image displayed between the driver and the front, it has been said that the driver can recognize the image without difficulty if the focus is formed about several meters ahead. Furthermore, extremely speaking, it is considered that, as long as the forward scene and the display information are superimposed and can be seen and in a case where both are in the field of view at the same time even if the display information were not in the same focal distance as the forward scene, the forward scene and the display information can be read out without having a feeling of incompatibility or fatigue so much, unlike looking at the displays of the meters of a normal instrument panel section.
If a complicated and fine image is displayed, then a suitable image display device, i.e., a CRT, a TFT transmission liquid display, etc., will be needed. In the present technique, it will be inevitable that the display device will become a projection type in order to display the image while superposing upon an external scene. In addition, in the projection type, in order for an image to be formed so as to have a focus far ahead, an optical system such as a lens or a mirror will become necessary. However, if it is assumed that a complicated image is not displayed unlike the aforementioned case, the limitation that a CRT is used in the image display section in order to make a projection type will be removed, and it will become possible to drastically change the concepts of the HUD.
As described above, in the automobiles, the space for housing an image display unit or an optical system is limited, and if possible, it is desirable that the use of this space is be avoided. A method of solving this problem is by using a transparent and flat image display unit to make a direct view type rather than the projection type, and by arranging this display unit in the vicinity of the windshield. If done like this, the driver can superimpose and look at the forward external scene and the display information at the same time and drive safely. At this time, what is necessary is that the image portion of this image display unit maintains a fixed brightness. A transparent electro-luminescent display can be simply used but is extremely expensive, and a passive image display unit is practical. However, an illumination system is needed in order for making use of this display unit, and this illumination system is also required to be transparent and flat.